Central Bureau for Astronomical Telegrams
IAUC 5176: N Mus 1991
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Circular No. 5176
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM EASYLINK 62794505
MARSDEN or GREEN@CFA.BITNET MARSDEN or GREEN@CFAPS2.SPAN
NOVA MUSCAE 1991
R. Sunyaev, E. Jourdain, and P. Laurent, on behalf of the
Granat and Gamma Teams: "GRS 1124-684 = GS 1124-683 has been
observed in x-ray and low-energy gamma-ray bands by the instruments
onboard the Granat Observatory on Jan. 9-10, 16-17, 17-18, and 20-21.
The source was localized by Sigma (at 30 keV) and ART-P (30 keV)
instruments at R.A. = 11h24m10s, Decl. = -68 24' (equinox 1950.0,
error circle 1'), in good agreement with the optical position (IAUC
5165). The Sigma telescope detected this source up to 200 keV. The
bremsstrahlung fit to the observed spectrum gives a temperature of
100 keV. The source intensity gradually declined twice between Jan.
9.9 and 10.9 UT. Hard x-ray luminosity in the 35- to 150-keV band
decreased from about 2 x 10E38 erg/s on Jan. 9-10 to 6 x 10E37 erg/s
during Jan. 16-21, when the source was relatively stable in this
band (the source distance was assumed to be 10 kpc). Variation of
intensity on a time scale of several hr was also found. WATCH All-
Sky Monitor data in the 12- to 60-keV band revealed gradual softening
of the spectrum. Some of the short-scale variations found by
WATCH were detected also by Sigma. The spectrum of the source
obtained on Jan. 9-10 at 30 keV is well described by the Comptonized
disk model (kT about 35 keV, optical depth about 1.2) or bremsstrahlung
of optically thin plasma (kT about 100 keV) with exponential
cut-off at higher energies. The spectrum gradually became harder,
and on Jan. 16-17 and 17-18 it can be described by a power law with
photon index about 2.2-2.3. The spectrum obtained on Jan. 20-21 is
seen up to about 600 keV without exponential cut-off and exhibits
spectral features at 100-200 and 400-500 keV; it looks similar to a
spectrum for the hardest gamma-ray bursts. On Jan. 21 the Pulsar
X-2 X-Ray Monitor aboard the Gamma spacecraft detected this source
in the 2- to 25-keV band. Its spectrum has a bright soft component.
The 2- to 6-keV flux was about 3 Crab, in comparison with 400 mCrab
in the 15- to 100-keV band, according to the Granat data. Analysis
of ART-P data in the 10- to 30-keV energy band did not reveal the
presence of low-frequency flickering noise, like that seen in Cyg
X-1 or GS 2023+338. The presence of a strong soft component and
absence of low-frequency noise make this source analogous to GS
2000+25, also exhibiting a hard tail (IAUC 4606). Granat and Gamma
will continue monitoring of this source until the middle of February."
1991 January 29 (5176) Daniel W. E. Green
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